1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic photoconductor and an image forming method. In particular, the invention relates to an image forming apparatus using a monolayer type electrophotographic photoconductor capable of coping with a wide variety of rotational speeds and an image forming method using the image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus used for printers, copying machines and the like adopts an image forming process in which charging means for charging an electrophotographic photoconductor, exposure means for exposing the charged photoconductor surface to form a latent image, developing means for transferring a toner to the latent image to develop it, transfer means for transferring the toner to a recording sheet to form an image, and electricity removing means for removing a residual potential remaining on the photoconductor surface after transfer are arranged in this order around a periphery of the electrophotographic photoconductor.
Here, the charging means can be roughly classified into charging means of a contact charging system for bringing a charging member such as a charging roller into direct contact with the surface of an electrophotographic photoconductor and charging means of a non-contact charging system for corona charging the surface of a photoconductor using a corona charger.
On the other hand, examples of organic photoconductors used include a photoconductor containing a charge generating agent (CGM) and a charge transfer agent (CTM) in a single photo sensitive layer, and a photoconductor produced by laminating a charge generating layer (CGL) and a charge transfer layer (CTL).
Stable charging properties of photoconductors are also divided into a negative charging property and a positive charging property. A negative charging photoconductor generates a large amount of ozone due to negative polarity corona, posing problems of an environmental pollution and a photoconductor deterioration due to ozone. Hence, since special discharging means or ozone decomposing means for preventing the problems from arising needs to be provided, a positive charging photoconductor is said to be excellent for a small size copying machine, facsimile, printer or the like.
As an image forming method of using such a positive charging photoconductor, a method is widely known that involves positively charging a positive charging type organic photoconductor by means of a corotron or scorotron and then carrying out a series of Carlson processes to form an image.
However, although a positive charging type organic photoconductor has the advantage of generating few amount of ozone during charging, the surface potential of the photoconductor gradually deteriorates along with repeated uses or long-term use, being liable to change also in image density.
In particular, when the rotational speed of an electrophotographic photoconductor is as high as 100 mm/sec or higher, both the contact and non-contact charging systems raise the problem of being prone to be large in the variation of a charging potential. This results in the problem that a partial charging failure is generated in an electrophotographic photoconductor, whereby fogging is easily generated in a formed image.
Under the circumstance, a method is disclosed in which in order to obtain a stable charging property even when charging means of a non-contact charging system is used in combination with a monolayer type electrophotographic photoconductor, a main charger provided with a charging member, a shield and a grid for a shield opening is used in main charging for the photoconductor, and the shield and the grid are maintained at a positive potential higher than zero while an inflow current (ISC+ICC) into the shield and grid is controlled to be 90% or less an inflow current (ICC) into the main charger (see, e.g. Patent document 1).
[Patent document 1] JP-8-278684A (Claims)
However, even if the inflow current is adjusted, there is a problem in that ozone or the like is prone to be generated when the rotational speed of the electrophotographic photoconductor is as high as 100 mm/sec or higher. This inhomogeneously oxidizes and deteriorates the surface of the electrophotographic photoconductor, thereby changing the properties of the electrophotographic photoconductor surface in some cases. Thus, in such a case, even the image forming member of Patent document 1 has a problem in that the variation of a charging potential cannot be sufficiently decreased.
Accordingly, an image forming apparatus is needed that is capable of effectively decreasing the variation of the charging potential even in the case of the contact charging system or non-contact charging system or further in the case where the rotational speed of an electrophotographic photoconductor is relatively high.